This invention relates generally to transmission of signals carried on a modulated light beam and, more particularly, to techniques for improving the performance characteristics of optical signal transmission systems. There are a number of applications that benefit from the use of an optical-frequency carrier for the transmission of data signals of various types. Optical communication signals are immune to electromagnetic interference and provide a very wide bandwidth. Moreover, optical signals may be conveniently transmitted through light-weight fibers. Fiber-optic communication links are already widely used for the transmission of digital data, and would also be of substantial value for analog signal transmission, except that the performance of such systems for analog signal transmission has significant limitations.
An important measure of the performance of a communication link is its dynamic range, which may be defined as the ratio of the largest signal that can be transmitted without harmonic distortion to the smallest signal that can be transmitted and still detected above a noise level inherent to the link. The dynamic range is usually expressed as a ratio of signal powers, in decibels (db). A major source of signal distortion is the process by which the analog signal to be transmitted is encoded as modulation onto the optical carrier signal. Intensity modulation of a light beam is typically performed by means of a Mach-Zehnder modulator. The modulated optical signal is then transmitted over an optical fiber. At a receiver, the optical signal is demodulated, usually by means of a photodetector. The transfer characteristic of a Mach-Zehnder modulator device is nonlinear over most of its range. More specifically, the transfer characteristic varies almost sinusoidally with applied voltage. The conventional approach to modulation using a Mach-Zehnder modulator has been to bias the device electrically to a near-linear region of the transfer characteristic curve. This bias point is usually referred to as the quadrature bias point. Use of this bias point has the advantage that second harmonics and all other even higher-order harmonics are virtually eliminated from the output of the communication link. Other forms of distortion remain, specifically two-tone intermodulation tones caused by interaction of two modulating signals of different frequencies, but these are lower in power than a second-harmonic component would be, so a larger maximum power signal can be transmitted without distortion, as compared with the maximum power that could be transmitted if second harmonic distortion were present.
The conventional approach to increasing dynamic range in an optical communication link has been to find a way to increase the maximum power that can transmitted without distortion. Using the quadrature bias point of a Mach-Zehnder modulator is consistent with this approach. It is widely believed that any further improvement in dynamic range can be obtained only by developing a light intensity modulator that has a linear transfer characteristic over a wide operating range, but such a perfectly linear modulator has yet to be developed.
The dynamic range of an optical communication link may also be increased by reducing the noise level inherent in the link and its associated components. One way to do this is to bias a Mach-Zehnder modulator at a different point in its transfer characteristic, referred to as the low-bias point. By operating at this bias point, the effective noise floor of the system is reduced without increasing the power of the two-tone intermodulation components. However, second-harmonic tones are generated in the output. The second harmonics can be easily filtered out, leaving the original modulating frequencies and the two-tone intermodulation components. The overall result is an increased dynamic range, but at the expense of bandwidth. If the modulating signals exceed one octave, some of the modulating frequencies will be lost in the filtering step needed to eliminate the second harmonics. Therefore, the system is limited to a sub-octave band of modulating signals.
Another approach to extending the dynamic range of externally modulated fiber-optic links is to cascade two Mach-Zehnder modulators and adjust the bias of both devices to eliminate both second- and third-order distortion. Any improvement is obtained at the expense of increased optical insertion loss, increased control complexity, and decreased bandwidth. Yet another approach is to adjust the polarization state of the light input to a Mach-Zehnder modulator. This also complicates bias control and is not a satisfactory solution.
It will be appreciated from the foregoing that there is still a need for further improvement in fiber-optic communication links for transmitting analog signals. In particular, what is needed is a technique for providing increased dynamic range even when the modulating signals extend over a multi-octave bandwidth. The present invention is directed to this end.